Quantum coding.

A theorem is proven for quantum information theory that is analogous to the noiseless coding theorem of classical information theory. In the quantum result, the von Neumann entropy S of the density operator describing an ensemble of pure quantum signal states is equal to the number of spin-1/2 systems (quantum bits or \qubits") necessary to represent the signal faithfully. The theorem holds whether or not the signal states are orthogonal. Related results are also presented about the delity of quantum coding and about representing entangled quantum states.